This application claims the priority of Japanese Patent Application No. 2001-177611 filed on Jun. 12, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color separating optical system mainly used in imaging apparatus such as TV cameras and video cameras. More specifically, it relates to a color separating optical system, constituted by a combination of a plurality of prisms and wavelength-selective filters, for decomposing incident light into three light components, i.e., a light component (hereinafter referred to as xe2x80x9cblue region lightxe2x80x9d) in a short wavelength region including blue light, a light component (hereinafter referred to as xe2x80x9cgreen region lightxe2x80x9d) in an intermediate wavelength region including green light, and a light component (hereinafter referred to as xe2x80x9cred region lightxe2x80x9d) in a long wavelength region including red light, and emitting thus obtained individual light components toward solid-state imaging devices and the like.
2. Description of the Prior Art
As this kind of color separating optical systems, various optical systems have conventionally been known, such as those known as Philips type and cross dichroic type according to characteristic features in how a plurality of prisms and color-separating optical filters (hereinafter simply referred to as xe2x80x9cfiltersxe2x80x9d) are combined.
In recent years, as imaging devices for converting captured optical images into electric signals in imaging apparatus such as TV cameras incorporating the above-mentioned kind of color separating optical systems, solid-state imaging devices such as CCD have recently come into use in place of image pickup tubes in general. Hence, it becomes easier to carry out a technique which has been difficult when using image pickup tubes which are likely to cause problems such as burn-in, i.e., a technique in which an object image having a high luminance is captured into a color separating optical system so as to be decomposed in terms of colors and then each color light component is emitted to the imaging surface of its corresponding solid-state imaging device while in a high luminance state.
However, the imaging surface of a solid-state imaging device has a high reflectivity in general since it is provided with a metal coating film and so forth. Therefore, light having a high luminance incident on the imaging surface may be reflected by the imaging surface, so as to return to the light exit face of the color separating optical system, and thus returned light is likely to be reflected by the light exit face and then enter the imaging surface of the solid-state imaging device again. Such a back-and-forth travel of light occurring between the light exit face of color separating optical system and the imaging surface of solid-state imaging device may generate ghost and flare, thereby adversely affecting images.
Recently, methods for suppressing such a back-and-forth travel of light in the green light region, in particular, have been proposed. For example, Japanese Unexamined Patent Publication No. 2000-98442 discloses a proposal for adjusting spectral characteristics of a trimming filter disposed at a green light exit face of a color separating optical system, so as to reduce the quantity of light reflected by the green light exit face, thereby restraining the back-and-forth travel of light from occurring between the green light exit face and the imaging surface of the solid-state imaging device.
In principle, such a back-and-forth travel of light is not limited to the green region light, but may occur in the red region light and blue region light as well. However, due to the sensitivity of solid-state imaging device and the like, it had hardly been regarded problematic except in the green region light. Nevertheless, technical improvements in solid-state imaging devices and the like have brought adverse affects caused by the above-mentioned back-and-forth travel of light into notice in the red region light as well, thereby enhancing a demand for suppressing this phenomenon.
Hence, a trimming filter may be disposed at the red light exit face of the color separating optical system, and spectral characteristics of the trimming filter may be adjusted so as to reduce the quantity of light reflected by the red light exit face, thereby suppressing the back-and-forth travel of red region light. However, this technique is hard to attain satisfactory results.
As a reason why the above-mentioned technique fails, it is presumed that, due to filters such as dichroic films disposed at prism surfaces in conventional color separating optical systems, the red region light is guided to the red light exit face while in a state where only the shorter wavelength side is cut, whereas the green region light is guided to the green light exit face while in a state where both shorter and longer wavelength sides are cut (see Japanese Unexamined Patent Publication No. HEI 11-38357 and the like). Namely, when suppressing the reflection at the red light exit face by cutting the longer wavelength side with a trimming filter disposed at the red light exit face, the amount of reflection at the red light exit face theoretically depends on the area of the overlapping part between the respective characteristic curves indicating transmission and reflection characteristics of the trimming filter because of the fact that the reflection characteristic of the trimming filter is the reverse of the transmission characteristic thereof. While the area of the above-mentioned overlapping part must be reduced in order to lower the amount of reflection at the red light exit face, spectral characteristics (transmission and reflection characteristics) of the trimming filter must be particularly excellent, i.e., substantially 100% at each wavelength, in order to attain sufficient results. Trimming filters having such excellent spectral characteristics are very hard to prepare, which makes it quite difficult to suppress the back-and-forth travel of light at the red light exit face.
In view of foregoing circumstances, it is an object of the present invention to provide a color separating optical system which can reduce the amount of reflection of return light from a solid-state imaging device at a red light exit face, thereby yielding favorable images free of ghost and flare.
For achieving the above-mentioned object, the present invention provides a color separating optical system comprising a plurality of prisms different from each other and a plurality of wavelength-selective filters having spectral characteristics different from each other for selectively transmitting or reflecting light in a predetermined wavelength range, the prisms and wavelength-selective filters separating light incident on an entrance face of the color separating optical system into a light component in a short wavelength region including blue light, a light component in an intermediate wavelength region including green light, and a light component in a long wavelength region including red light and emitting thus obtained individual light components from blue, green, and red light exit faces, respectively; the color separating optical system comprising a first filter, disposed at a prism surface other than the red light exit face in an optical path for guiding the light component in the long wavelength region including the red light, having a first wavelength characteristic adapted to eliminate from light emitted from the red light exit face a light component in a wavelength range longer than a predetermined longer-wavelength-side boundary wavelength of a red visible light wavelength region; and a second filter, disposed at the red light exit face, having a second wavelength characteristic adapted to reflect a light component in a wavelength range longer than a second boundary wavelength longer than the longer-wavelength-side boundary wavelength by a predetermined wavelength but transmit therethrough a light component in a wavelength region shorter than the second boundary wavelength.
Preferably, the first filter is a filter having a characteristic adapted to eliminate from the light emitted from the red light exit face a light component in a wavelength range shorter than a predetermined shorter-wavelength-side boundary wavelength of the red visible light wavelength region and the first characteristic.
Such a first filter may be constituted by a dichroic film having a spectral characteristic of reflecting a light component within a wavelength range from the shorter-wavelength-side boundary wavelength to the longer-wavelength-side boundary wavelength.
Preferably, the longer-wavelength-side boundary wavelength is set within a range from 680 nm to 730 nm, whereas the difference between the longer-wavelength-side boundary wavelength and the second boundary wavelength is set within a range from 15 nm to 70 nm.
The color separating optical system may comprise a color separating prism of Philips type or cross dichroic type.